Start zygote with -Xopaque-jni-ids:swapable

This allows the use of structural class redefinition on debuggable
apps.

Also allows override of this via the new 'dalvikvm.vm.opaque-jni-ids'
option.

Test: Manual
Test: ./art/tools/jvmti-agents/field-counts/count-fields.py 'Ldalvik/system/ClassExt;.instanceJfieldIDs:Ljava/lang/Object;' 'Ldalvik/system/ClassExt;.staticJfieldIDs:Ljava/lang/Object;' 'Ldalvik/system/ClassExt;.jmethodIDs:Ljava/lang/Object;'
Bug: 134162467
Change-Id: I9e9c7e5a7ff95d62f9969ebeb472e81a7357d7f9